Grafted Polymer Surfaces for Dropwise Condensation, and Associated Methods of Use and Manufacture

ABSTRACT

Presented herein are articles and methods featuring substrates with thin, uniform polymeric films grafted (e.g., covalently bonded) thereupon. The resulting coating provides significant reductions in thermal resistance, drop shedding size, and degradation rate during dropwise condensation of steam compared to existing coatings. Surfaces that promote dropwise shedding of low-surface tension condensates, such as liquid hydrocarbons, are also demonstrated herein.

RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional applicationSer. No. 14/620,661, filed Feb. 12, 2015, which is a continuation ofU.S. Non-Provisional patent application Ser. No. 14/181,586, filed onFeb. 14, 2014, which claims priority to and the benefit of U.S.Provisional Patent Application No. 61/876,195, filed Sep. 10, 2013, U.S.Provisional Patent Application No. 61/874,941, filed Sep. 6, 2013, andU.S. Provisional Patent Application No. 61/765,679, filed Feb. 15, 2013.

GOVERNMENT SUPPORT

This invention was made with government support under Contract No.W911NF-07-D-0004 awarded by the Army Research Office. The government hascertain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to grafted polymer surfaces and theiruse for enhanced heat transfer, improved dropwise condensation, and/orreduced adhesion of liquids and solids thereto.

BACKGROUND OF THE INVENTION

Vapors condense upon a surface if the surface is cooled below thesaturation temperature at a given pressure. The condensed liquid phasemay accumulate on the surface as a film and/or as droplets or islands ofliquid. Condensation is critical in many industrial applications,although in certain applications, it is useful to inhibit or prevent thefilmwise buildup of condensing liquid on a surface by promoting dropletshedding and enhancing dropwise condensation.

Condensation of water is a crucial process in many industries, includingpower generation and desalination. Roughly 85% of the global installedbase of electricity generation plants and 50% of desalination plantsworldwide rely on steam surface condensers, a type of heat exchanger inwhich a plurality of tubes flowing coolant contact steam on theiroutside surface. Given the widespread scale if these processes, evenslight improvements in cycle efficiencies will have a significant effecton global energy consumption.

One useful measure of heat transfer performance for a condenser is theheat transfer coefficient, defined as the flux per area in units ofkW/m²K. Heat transfer coefficients experienced when condensing in thedropwise mode are an order of magnitude greater than those in thefilmwise mode. The presence of an insulating liquid film during filmwisecondensation presents a significant thermal barrier to heat transfer,whereas the departure of discrete drops during dropwise condensationexposes the condensing surface to vapor. The higher heat transfercoefficients experienced during dropwise condensation make it attractivefor employing in large-scale thermal fluids applications such as steampower plants and desalination plants, as well as small-area high-heatflux applications such as electronics cooling. However, the practicalimplementation of dropwise condensation in power generation,desalination, and other applications has been a significant materialschallenge, limited by, among other factors, durability of existinghydrophobic functionalization for metal heat transfer surfaces. Whilemetals provide both high thermal conductivity for maximizing heattransfer and high tensile strength to minimize the need for structuralsupports, metals are typically wetted by water and most other thermalfluids, and, as a result, metals exhibit filmwise condensation. In orderfor a metal surface to exhibit desired dropwise condensation, thesurface that is used for heat transfer needs to be modified. One way toachieve dropwise condensation on a metal surface where heat transfertakes place is to modify the metal surface with a hydrophobic coating.

A number of conventional techniques have been employed previously topromote dropwise condensation on surfaces, including the use ofmonolayer promoters such as oleic acid and stearic acid (U.S. Pat. No.2,919,115), noble metals (U.S. Pat. No. 3,289,753 and U.S. Pat. No.3,289,754 and U.S. Pat. No. 3,305,007), ion-implanted metal (U.S. Pat.No. 6,428,863), as well as thin films of polymers applied via sputteringor dip-coating (U.S. Pat. No. 2,923,640, U.S. Pat. No. 3,899,366,EP2143818 Al, U.S. Pat. No. 3,466,189). However, previous methods sufferfrom problems such as low durability and/or high cost. Moreover, most ofthese hydrophobic modifiers, and especially the silane-based modifiersthat have been used in some conventional methods, are not robust insteam environments of industrial interest (in other words, thesemodifiers cannot withstand the environments in which they are used).Previous methods also do not adequately promote rapid droplet sheddingbecause they do not sufficiently reduce the contact angle hysteresis. Itis possible to have a surface with a high contact angle but also highadhesion, so even though condensation would initiate in the dropwiseregime, it would ultimately progress to filmwise condensation becausethe drops are not able to shed easily.

Furthermore, where the condensing liquids are hydrocarbons or otherlow-surface tension liquids, the problem of film-wise condensation isexacerbated. Current surfaces designed for dropwise condensation ofwater do not promote dropwise condensation of low-surface tensionhydrocarbon liquids such as n-alkanes (e.g., octane, hexane, heptane,pentane, butane) and refrigerants (e.g., fluorocarbons,chlorofluorocarbons, hydrochlorofluorocarbons) and cryogenic liquids(e.g., LNG, O2, N2, CO2, methane, propane).

Some conventional methods have used nanotextured surface to improvecondensation heat transfer, however, these methods also rely on silaneor thiol modifiers to modify the wettability of a nanotextured surfacefrom superhydrophilic to superhydrophobic, and thus these nanotexturedsurfaces are subject to the same robustness concerns discussed above.Additionally, because the thermal conductivities of polymeric materialsare typically orders of magnitude smaller than those of a typical metalsubstrate, the thickness of the polymer modifier is extremely important.Hence, there is currently a need for an ultra-thin robust hydrophobicmodifier that may be applied over a metal surface to enhance heattransfer.

There is a need for methods and articles/devices for improved heattransfer and/or dropwise condensation of low-surface tension liquids,including hydrocarbon liquids.

SUMMARY OF THE INVENTION

Presented herein are articles and methods featuring substrates withthin, uniform polymeric films grafted thereupon. Techniques such as iCVDallow deposition of precisely-controlled, extremely thin polymeric filmson metal substrates, where the polymer is covalently bonded to thesubstrate. Furthermore, the polymeric film may be crosslinked at or nearits exposed surface and/or throughout the bulk of the film viaannealing. The resulting coating exhibits significant reductions inthermal resistance, drop shedding size, and degradation rate duringdropwise condensation of steam compared to existing coatings.

Articles and methods presented herein relate to the use of ecofriendlymonomers (e.g., 1H, 1H, 2H, 2H—perfluorooctyl acrylate (C6)) for iCVD.C6 monomers undergo surface group reorganization, which is undesirable.Articles and methods presented herein relate to overcoming the surfacegroup organization via crosslinking and/or graded structure. In someembodiments, 1H, 1H, 2H, 2H—perfluorooctyl acrylate as well as C6monomers with alternative chemistries are deposited via iCVD asprecisely-controlled, extremely thin polymeric films on metalsubstrates, where the polymer becomes covalently bonded to thesubstrate.

In some embodiments, the invention relates to an article for enhancedheat transfer, and/or mitigating phase transition and nucleation ofundesired materials, and/or reducing adhesion of liquids and solidsthereupon, the article comprising a substrate and a (e.g., thin,uniform) polymeric film grafted (e.g., covalently bonded) thereupon.

In some embodiments, the substrate comprises a metal (e.g., steel,stainless steel, titanium, nickel, copper, aluminum, molybdenum, and/oralloys thereof). In some embodiments, the substrate comprises a polymer(e.g., polyethylene, polyvinylchloride, polymethylmethacrylate,polyvinylidene fluoride, polyester, polyurethane, polyanhydride,polyorthoester, polyacrylonitrile, polyphenazine, polyisoprene,synthetic rubber, polytetrfluoroethylene, polyethylene terephthalate,acrylate polymer, chlorinated rubber, fluoropolymer, polyamide resin,vinyl resin, expanded polytetrafluoroethylene, low density polyethylene,high density polyethylene, and/or polypropylene). In some embodiments,the substrate comprises a semiconductor and/or ceramic (e.g., SiC, Si,AlN, GaAs, GaN, ZnO, Ge, SiGe, BN, BAs, AlGaAs, TiO₂, TiN, etc.). Insome embodiments, the substrate comprises a rare earth element orcompound comprising a rare earth element (e.g., a rare earth oxide,carbide, nitride, fluoride, or boride; e.g., cerium oxide CeO₂).

In some embodiments, the polymeric film comprises a fluoropolymer. Insome embodiments, the polymeric film is formed from at least one monomerspecies comprising one or more pendant perfluorinated alkyl moieties. Insome embodiments, the fluoropolymer has at least one CF₃ group. In someembodiments, the fluoropolymer comprises polytetrafluoroethylene (PTFE).In some embodiments, the fluoropolymer comprises [C₁₂H₉F₁₃O₂]_(n), wheren is an integer greater than zero.

In some embodiments, the fluoropolymer comprises a member selected fromthe group consisting ofpoly(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl methacrylate),poly(1H,1H,2H,2H-perfluorooctyl acrylate),poly([N-methyl-perfluorohexane-1-sulfonamide] ethyl acrylate),poly([N-methyl-perfluorohexane-1-sulfonamide] ethyl (meth) acrylate),poly(2-(Perfluoro-3-methylbutypethyl methacrylate)),poly(2-[[[[2-(perfluorohexyl)ethyl]sulfonyl]methyl]-amino]ethyl]acrylate),poly(2-[[[[2-(perfluoroheptyl)ethyl]sulfonyl]methyl]-amino]ethyl]acrylate),poly(2-[[[[2-(perfluorooctyl)ethyl]sulfonyl]methyl]-amino]ethyl]acrylate),and any copolymer thereof.

In some embodiments, the fluoropolymer is a C6 analog of PFDA. In someembodiments, the fluoropolymer comprisespoly(2-(Perfluoro-3-methylbutyl)ethyl methacrylate), or any copolymercomprising 2-(Perfluoro-3-methylbutyl)ethyl methacrylate, wherein thefluoropolymer is crosslinked.

In some embodiments, the polymeric film comprises at least one memberselected from the group consisting of polytetrafluoroethylene (PTFE),poly(perfluorodecylacrylate) (PFDA), polymethylmethacrylate (PMMA),polyglycidylmethacrylate (PGMA), poly-2-hydroxyethylmethacrylate,poly(perfluorononyl acrylate), poly(perfluorooctyl acrylate), and anycopolymer thereof. In some embodiments, the polymeric film comprises acopolymer of two or more monomer species.

In some embodiments, the polymeric film comprises cross-linked polymerand/or cross-linked copolymer. In some embodiments, the polymeric filmis cross-linked with a crosslinking agent comprising an organic moleculehaving at least two vinyl moieties. In some embodiments, the polymericfilm is cross-linked with a crosslinking agent comprising at least onemember selected from the group consisting of: diethyleneglycol divinylether, diethyleneglycol dimethacrylate, diethyleneglycol diacrylate,and/or 1H,1H,6H,6H-perfluorohexyldiacrylate. In some embodiments, thepolymeric film is cross-linked with divinyl benzene (DVB). In someembodiments, the polymeric film is cross-linked with a member selectedfrom the group consisting of ethylene dimethyacrylate (EDMA),di(ethyleneglycol)di(methacrylate), di(ethyleneglycol)di(acrylate),ethyleneglycoldimethyacrylate (EGDMA), di(ethyleneglycol)di(vinylether)(EDGDVE), and 1H,1H,6H,6H-perfluorohexyldiacrylate.

In some embodiments, the polymeric film comprises from 0 wt. % to 99 wt.% crosslinking agent (e.g., from 5 wt. % to 90 wt. %; from 15 wt. % to85 wt. %; from 25 wt. % to 75 wt. %; from 35 wt. % to 65 wt. %; or from45 wt. % to 55 wt. %).

In some embodiments, the polymeric film has non-uniform concentration ofcrosslinking agent along the thickness of the film. In some embodiments,the polymeric film is covalently bonded to the substrate. In someembodiments, the polymeric film is covalently bonded to the substrate byattachment of a vinyl precursor to the substrate, thereby forming asurface comprising a plurality of pendant vinyl moieties. In someembodiments, the vinyl precursor is a member selected from the groupconsisting of a vinyl functional silane, a vinyl functional phosphonicacid, and a vinyl functional thiol.

In some embodiments, the vinyl precursor comprises at least one memberselected from the group consisting of trichlorovinyl silane,bis(triethoxysilylethyl)vinylmethyl-silane, bis(triethoxysilyl)ethylene,bis(trimethoxysilylmethyl)ethylene,1,3-[bis(3-triethoxysilylpropyl)poly-ethylenoxy]-2-methylenepropane,bis[(3-trimethoxysilyl)propyl]-ethylenediamine,bis[3-(triethoxysilyl)propyl]-di sulfide,3-mercaptopropyltrimethoxysilane, and vinyl phosphonic acid.

In some embodiments, the polymeric film is no greater than 500 nm inthickness (e.g., no greater than 400 nm, no greater than 300 nm, nogreater than 200 nm, no greater than 100 nm, no greater than 75 nm, nogreater than 50 nm, no greater than 25 nm, or no greater than 15 nm,e.g., as thin as 10 nm). In some embodiments, the polymeric filmcomprises a grafting layer (e.g., where the polymeric film is covalentlybonded to the substrate) and a bulk film layer (e.g., where the graftinglayer has a thickness from about 0.5 nm to about 5 nm or from about 1 nmto about 3 nm, or from about 1 nm to about 2 nm). In some embodiments,the polymer film has a thickness variation of no greater than about 20%(e.g., no greater than about 15%, no greater than about 10%, or nogreater than about 5%—e.g., the polymer film is uniform).

In some embodiments, the polymer film has a texture comprising micro-and/or nano-scale features (e.g., ridges, grooves, pores, posts, bumps,and/or protrusions, patterned and/or unpatterned). In some embodiments,the substrate is textured and wherein the polymeric film conforms to thetextured substrate surface. In some embodiments, the substrate istextured with micro- and/or nano-scale surface textures (e.g., posts,ridges, cavities, pores, posts, protrusions, etc.). In some embodiments,the polymeric film has a crystalline or semicrystalline surface (e.g.,formed via annealing, but not necessarily via annealing).

In some embodiments, the polymeric film has a surface (e.g., exposedsurface) with low contact angle hysteresis (e.g., no greater than 50°,no greater than 40°, no greater than 30°, no greater than 25°, nogreater than 20°, no greater than 15°, or no greater than 10°, or nogreater than 5°, or no greater than 1° for water; and no greater than20°, no greater than 15°, no greater than 10°, no greater than 5°, or nogreater than 1° for hydrocarbons, refrigerants, cryogenic liquids, andother low-surface tension liquids, where contact angle hysteresis is thedifference between advancing contact angle and receding contact angle).

In some embodiments, the polymeric film has a surface (e.g., exposedsurface) with high advancing contact angle (e.g., no less than 70°, noless than 80°, no less than 90°, no less than 100°, no less than 120°,no less than 130°, no less than 140° for water; and no less than 30°, noless than 40°, no less than 50°, no less than 60°, no less than 70°, noless than 80°, no less than 90°, no less than 100° for hydrocarbons,refrigerants, cryogenic liquids, and other low-surface tension liquids)and/or high receding contact angle (e.g., no less than 60°, no less than70°, no less than 80°, no less than 90°, no less than 100°, no less than110°, or no less than 120° for water; and no less than 20°, no less than30°, no less than 40°, no less than 50°, no less than 60°, no less than70°, no less than 80°, no less than 90° for hydrocarbons, refrigerants,cryogenic liquids, and other low-surface tension liquids).

In some embodiments, the article is a condenser (e.g., where dropwisecondensation is promoted on the surface of the polymeric film forenhanced heat transfer). In some embodiments, the article is a coolingdevice for an electronic and/or photonic component (e.g., where heattransfer is promoted from the electronic or photonic component to thesurface of the polymeric film, wherein the polymeric film is in contactwith the component, and/or wherein the polymeric is in contact with afluid that is in contact with the component).

In some embodiments, the article is flexible. In some embodiments, thesubstrate and the polymeric film grafted thereupon is flexible. In someembodiments, the article is retrofitted to form the grafted polymericfilm.

In another aspect, the invention is directed to a method for using thearticle described in any of the above embodiments, wherein the methodcomprises contacting an exposed surface of the polymeric film with aThermal Interface Material (TIM) (e.g., a thermally conductive materialused between microprocessors and heatsinks to increase thermal transferefficiency).

In some embodiments, the polymeric film comprises a polymer and/orcopolymer, the polymer and/or copolymer comprising at least oneperfluorinated pendant chain (e.g., a perfluorinated acrylate and/or aperfluorinated cyclic group, e.g., with 4 to 6 carbons in the ring), aspacer group, and a vinyl-based backbone group.

In some embodiments, the method includes contacting an exposed surfaceof the polymeric film with a Thermal Interface Material (TIM) (e.g., athermally conductive material used between microprocessors and heatsinksto increase thermal transfer efficiency).

In some embodiments, the invention is directed to a method of preparingan article (e.g., the article described in any of the aboveembodiments), the method including the step of performing hot wire CVD(HWCVD) to produce the polymeric film grafted on the substrate. In someembodiments, the step of performing HWCVD comprises performing initiatedchemical vapor deposition (iCVD) to produce the polymeric film graftedon the substrate.

In some embodiments, the method further includes the step of annealingthe polymeric film by exposure to heat (e.g., to increase crosslinkingdensity and/or degree of crystallinity of the polymeric film). Inaddition, in some embodiments, annealing can reduce hysteresis, increasecrystallinity at interface, and increase crosslinking at the exposedinterface.

In some embodiments, the HWCVD step is performed to retrofit an existingarticle (e.g., a condenser, boiler or other heat transfer surface in anHVAC device, a power plant, a desalination plant, a natural gasliquefaction ship, etc.) by grafting the polymeric film upon a surfacethereof.

In some embodiments, the article is a Thermal Interface Material (TIM).

In some embodiments, the polymeric film has an exposed surface withcritical surface energy no greater than 18 mN/m. In some embodiments,the polymeric film has an exposed surface with critical surface energyno greater than 6 mN/m.

In some embodiments, the polymeric film has an exposed surface withcontact angle hysteresis no greater than 25° for water, hydrocarbons,refrigerants, cryogenic liquids, and other heat transfer fluids. In someembodiments, the exposed surface has contact angle hysteresis no greaterthan 1° or no greater than 5° for water, hydrocarbons, refrigerants,cryogenic liquids, and other heat transfer fluids.

In some embodiments, the polymeric film has RMS roughness no greaterthan 100 nm (e.g., no greater than 100 nm, no greater than 75 nm, nogreater than 50 nm).

In some embodiments, the polymeric film provides dropwise condensationand shedding of a hydrocarbon, refrigerant, cryogenic liquid, water, orother low-surface tension liquid. In some embodiments, the hydrocarbonliquid is a member selected from the group consisting of alkanes,alkenes, alkynes, and fuel mixtures (e.g., gasoline, kerosene, diesel,fuel oil); the refrigerant is a member selected from the groupconsisting of chlorofluorocarbons, hydrofluorocarbons, andhydrochlorofluorocarbons; and the cryogenic liquid is selected from thegroup consisting of N₂, O₂, CO₂, He, LNG, methane, butane, propane, andisobutene. In certain embodiments, the hydrocarbon liquid is a memberselected from the group consisting of hexane, toluene, isopropanol,ethanol, octane, pentane, and perfluorohexane.

In some embodiments, the hydrocarbon liquid has surface tension nogreater than 30 mN/m (e.g., no greater than 28 mN/m, no greater than 21mN/m, no greater than 18 mN/m, no greater than 16 mN/m, or no greaterthan 12 mN/m, or no greater than 6 mN/m).

In some embodiments, the article is a component (e.g., vessel, pipe,fin, etc.) of a condenser that comes into contact with a condensingliquid (e.g., working fluid). In some embodiments, the article is acomponent of an oil and/or gas processing apparatus (e.g. fractionationcolumn, liquefaction device). In some embodiments, the article is (or isa component of) a power line, a turbine, an aircraft, a pipeline, aboiler, a windshield, a solar panel, industrial machinery, cookware, aconsumer electronic device, a printed circuit board, an electroniccomponent, or a medical device.

Another aspect discussed herein relates to a method for manufacturing asurface for promoting dropwise condensation and/or shedding of a liquid,the method including the steps of: providing a substrate; andcontrollably depositing a polymeric film on the substrate via initiatedchemical vapor deposition (iCVD).

In some embodiments, the method includes depositing a vinyl precursor onthe substrate prior to, or concurrently with, depositing the polymericfilm. In some embodiments, the method includes modulating an averageroughness of the deposited layer (e.g., such that roughness is nogreater than 100 nm, or no greater than 75 nm, or no greater than 50nm). In some embodiments, the modulating includes monitoring a degree ofcrystallization of the deposited polymeric film; or controlling theproportion of crosslinker; or controlling a temperature of the substrateduring deposition; or any combination thereof.

In some embodiments, the deposited polymeric film has an averagethickness from 1 nm to 1 micron. In some embodiments, the depositedpolymeric film has an average thickness from 1 nm to 100 nm.

In some embodiments, the substrate comprises one or more materialsselected from the group consisting of a metal (e.g., copper, brass,stainless steel, aluminum, aluminum bronze, nickel, iron, nickel ironaluminum bronze, titanium, scandium, and any alloys thereof), polymer,glass, rubber, silicon, polycarbonate, PVC, ceramic, semiconductor, andany combinations thereof. In some embodiments, the substrate comprisesone or more materials selected from the group consisting of plastic,silicon, quartz, woven or non-woven fabric, paper, ceramic, nylon,carbon, polyester, polyurethane, polyanhydride, polyorthoester,polyacrylonitrile, polyphenazine, polyisoprene, synthetic rubber,polytetrfluoroethylene, polyethylene terephthalate, acrylate polymer,chlorinated rubber, fluoropolymer, polyamide resin, vinyl resin,expanded polytetrafluoroethylene, low density polyethylene, high densitypolyethylene, and polypropylene.

In some embodiments, the polymeric film has an exposed surface withcritical surface energy no greater than 18 mN/m. In some embodiments,the polymeric film has an exposed surface with critical surface energyno greater than 6 mN/m.

In some embodiments, the polymeric film has an exposed surface withcontact angle hysteresis no greater than 25°. In some embodiments, theexposed surface has contact angle hysteresis no greater than 5° forwater, hydrocarbons, refrigerants, cryogenic liquids, and other heattransfer fluids, or any combination thereof.

In some embodiments, the polymeric film has roughness no greater than100 nm (e.g., no greater than 100 nm, no greater than 75 nm, no greaterthan 50 nm).

In some embodiments, the polymeric film provides dropwise condensationand shedding of a hydrocarbon, refrigerant, cryogenic liquid, water,other low-surface tension liquids, or any combination thereof. In someembodiments, the hydrocarbon liquid is a member selected from the groupconsisting of alkanes, alkenes, alkynes, and fuel mixtures (e.g.,gasoline, kerosene, diesel, fuel oil); the refrigerant is a memberselected from the group of chlorofluorocarbons, hydrofluorocarbons, andhydrochlorofluorocarbons; and the cryogenic liquid is selected from thegroup consisting of N₂, O₂, CO₂, LNG, methane, propane, isobutene, andany combination thereof. In some embodiments, the hydrocarbon liquid hassurface tension no greater than 30 mN/m (e.g., no greater than 28 mN/m,no greater than 21 mN/m, no greater than 18 mN/m, no greater than 16mN/m, or no greater than 12 mN/m, or no greater than 6 mN/m).

Another aspect discussed herein relates to a method of manufacturing thepolymeric film on the article of any of the aspects or embodimentsdiscussed above.

Elements of embodiments described with respect to a given aspect of theinvention may be used in various embodiments of another aspect of theinvention. For example, it is contemplated that features of dependentclaim depending from one independent claim can be used in apparatus,articles, systems, and/or methods of any of the other independentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent and may be better understood byreferring to the following description taken in conjunction with theaccompanying drawings, in which:

FIGS. 1a-1c illustrate iCVD reactor geometries and reaction processes,in accordance with certain embodiments of the invention. FIG. 1billustrates a schematic of a lab-scale 200 mm diameter iCVD reactorsystem. For a vinyl homopolymerization, a constant flow of monomer andinitiator are metered into the ‘pancake’ style vacuum reaction chamber.An array of resistively heated wires, suspended a few centimeters abovethe substrate, heats the vapors. Laser inteferometry provides real timemonitoring of the iCVD polymer thickness. The pressure of the chamber iscontrolled by a throttling value. An unreacted species and volatilereaction by-products are exhausted to a mechanical pump. Forcopolymerization, an additional monomer feed line would need to be addedto the system. FIG. 1c shows a schematic cross-section of the iCVDreactor showing decomposition of the initiator by the heated filaments.Surface modification through polymerization of the monomer occurs on theactively cooled substrate.

FIGS. 2a-2e illustrate comparison of water condensation onp(PFDA-co-DVB) and fluorosilane coatings deposited on siliconsubstrates, in accordance with some embodiments of the presentinvention. Environmental scanning electron micrograph of condensation ofpure saturated water vapor at 800 Pa and a supersaturation of 1.16±0.05,showing pre-coalescence behavior on copolymer (FIG. 2a ) and comparingto condensation behavior on fluorosilane (FIG. 2b ) surfaces, indicatinghigher nucleation density on copolymer surface. Photographs ofcondensation of water vapor in air at 40% R.H. on copolymer (FIG. 2c )and fluorosilane surfaces (FIG. 2d ) immediately before and after ashedding event (top and middle photographs, respectively) and 15 secondsafter the shedding event (bottom photograph), indicating smallerdeparting drop diameter on copolymer surface. FIG. 2(e) illustratestime-averaged normalized droplet diameter distributions. Smaller dropsizes on copolymer surface indicate better shedding behavior.

FIGS. 3a-3e illustrate surface topology and water vapor condensation onp(PFDA-co-DVB) coating deposited on an aluminum substrate, in accordancewith some embodiments of the present invention. FIG. 3a illustrates50×50 μm AFM height scan of surface topology. Dashed box indicatesregion of the image shown in FIG. 3 b, 10×10 μm AFM height scan ofsurface topology. Photographs during condensation of saturated steam at100° C. and 101 kPa of prolonged dropwise condensation on graftedcoating over a period of 48 hours (FIG. 3c ) and degradation offluorosilane coating over a period of 30 min (FIG. 3d ). FIG. 3eillustrates heat transfer coefficient of aluminum substrates with nocoating, with a fluorosilane coating, and with a grafted p(PFDA-co-DVB)coating, plotted vs. time.

FIG. 4a illustrates dropwise condensation of saturated steam at 6.9 kPaon a copper tube coated with p(PFDA-co-DVB). FIG. 4b illustratessnapshots immediately before and after a droplet shedding event (leftand center photographs, respectively) and 4 hours after shedding event(right photograph).

FIG. 5 (left) illustrates high resolution angle-resolved XPS spectrataken at 0° takeoff angle. Peaks corresponding to —CF₂— and —CF₃environments are highlighted. FIG. 5 (right) illustrates 10×10 μm AFMheight scan of surface topology showing spherulitic texture. Dashed boxindicates region of (e), 1×1 AFM phase scan of single roughness feature(bottom) and line height scan (top).

FIG. 6 illustrates the experimental chamber that was used in DropwiseCondensation Experiments.

FIG. 7 illustrates the flow loop of the experimental setup shown in FIG.6.

FIG. 8a illustrates grafted PFDA samples after 1 hour of condensation insaturated steam at 90° C. and 70 kPa. FIG. 8b illustrates ungrafted PFDAsample after 1 hour of condensation in saturated steam at 90° C. and 70kPa. Condensate drops on grafted (FIG. 8c ) and ungrafted (FIG. 8d )PFDA surfaces after 10 minutes of condensing saturated steam. Thedistorted drop shape on the ungrafted sample indicates severe contactline pinning. Departing drop sizes on ungrafted sample were 3.1 mm,compared to 2.3 mm for the grafted surface. Heat transfer coefficientwas measured at 31±2 kW/m²K at the beginning of the experiment, and 23±2kW/m²K after deterioration of ungrafted surface.

FIG. 9 shows AFM images of the poly(1H,1H,2H,2H-perfluorodecyl Acrylate)(pPFDA)homopolymer and different poly(1H,1H,2H,2H-perfluorodecylAcrylate-copolymer-divylbenzene) (p(PFDA-co-DVB)) copolymers before andafter annealing. The flow rate for each monomer is provided between thebrackets.

FIG. 10 is a graph of FT-IR of the pPFDA homopolymer, the pDVBhomopolymer and a P(PFDA-co-DVB) copolymer in accordance with someembodiments presented herein.

FIG. 11 shows contact angle measurements using water and mineral oil ofthe pPFDA homopolymer, the pDVB homopolymer and a series ofp(PFDA-co-DVB) copolymers, for non-annealed (left) and annealed (right)samples.

FIG. 12 is a water contact angle graph for pPFDA homopolymer and aseries of p(PFDA-co-DVB) copolymers, for non-annealed (solid) andannealed samples (open) in accordance with some embodiments presentedherein.

FIG. 13 is a mineral oil contact angle graph for pPFDA homopolymer and aseries of p(PFDA-co-DVB) copolymers, for non-annealed (solid) andannealed samples (open) in accordance with some embodiments presentedherein.

FIG. 14 illustrates XPS analysis of different p(PFDA-co-DVB) copolymersin accordance with some embodiments presented herein.

FIG. 15 illustrates XRD analysis of the pPFDA homopolymer, the pDVBhomopolymer and diverse P(PFDA-co-DVB) copolymer in accordance with someembodiments presented herein.

FIG. 16 illustrates XRD analysis of the pPFDA homopolymer, the pDVBhomopolymer and diverse P(PFDA-co-DVB) copolymer after the annealingprocess in accordance with some embodiments presented herein.

FIG. 17 shows XRD Comparison of pPFDA homopolymer and a series ofp(PFDA-co-DVB) copolymer films before and after thermal annealing inaccordance with some embodiments presented herein.

FIG. 18 schematically illustrates embodiments employing variation indegree of crosslinking and/or variation in concentration of crosslinkingagent as a function of position of the crosslinking agent along thethickness of the polymeric film, in accordance with some embodimentspresented herein.

FIG. 19 is a plot showing the effect of contact angle θ on heat transfercoefficient h, where maximum h is at 0˜90° (hexane 66.5/54.6°), inaccordance with some embodiments presented herein.

FIGS. 20a and 20b are photographic stills from a video showing dropwisecondensation and shedding of n-hexane on a PFDA-co-DVB on siliconsubstrate, where P=15 kPa, T_(s)=10±1° C., T_(sat)=18.3° C., andΔT=8.3±1° C., in accordance with some embodiments presented herein.

FIG. 21 is a plot showing FT-IR spectra of eco-friendly pC6PFAhomopolymer (a), the pDVB homopolymer (b), and the p(C6PFA-co-DVB)copolymer.

FIG. 22 is a plot showing the water contact angles on PTFE, PVDF, anddiverse compositional ranges from pC6PFA homopolymer to pDVBhomopolymer.

FIG. 23 shows the reorientation, or lack thereof, of pendantperfluorinated pendant groups upon exposure with water: (a) demonstrateshow amorphous chains of C6 polymer reorient into the bulk and contributeto high CAH; and (b) shows how steric hindrance afforded by DVBcrosslinking restricts rearrangement of pendant groups into the bulk ofthe film, reducing CAH.

FIG. 24 is a plot of the advancing and receding contact angles, alongwith the contact angle hysteresis, of eco-friendly small-chainperfluorinated films, in accordance with some embodiments presentedherein.

The features and advantages of the present disclosure will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements.

DESCRIPTION

It is contemplated that compositions, mixtures, systems, devices,methods, and processes of the claimed invention encompass variations andadaptations developed using information from the embodiments describedherein. Adaptation and/or modification of the compositions, mixtures,systems, devices, methods, and processes described herein may beperformed by those of ordinary skill in the relevant art.

Throughout the description, where articles, devices and systems aredescribed as having, including, or comprising specific components, orwhere processes and methods are described as having, including, orcomprising specific steps, it is contemplated that, additionally, thereare articles, devices, and systems of the present invention that consistessentially of, or consist of, the recited components, and that thereare processes and methods according to the present invention thatconsist essentially of, or consist of, the recited processing steps.

Similarly, where articles, devices, mixtures, and compositions aredescribed as having, including, or comprising specific compounds and/ormaterials, it is contemplated that, additionally, there are articles,devices, mixtures, and compositions of the present invention thatconsist essentially of, or consist of, the recited compounds and/ormaterials.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the invention remains operable.Moreover, two or more steps or actions may be conducted simultaneously.

The mention herein of any publication, for example, in the Backgroundsection, is not an admission that the publication serves as prior artwith respect to any of the claims presented herein. The Backgroundsection is presented for purposes of clarity and is not meant as adescription of prior art with respect to any claim.

Presented herein are articles and methods featuring substrates withthin, uniform polymeric films grafted thereupon. An exposed surface ofthe film is configured for contact with a liquid, another solid, avapor, and/or a combined vapor and liquid—that is, there is either asolid-liquid interface, a solid-solid interface, solid-vapor interface,or a solid-vapor/liquid interface at the surface of the graftedpolymeric film. The polymeric film may be tuned to have a precisethickness and uniformity. For example, a thickness of less than about200 nm, less than about 150 nm, less than about 100 nm, less than about80 nm, less than about 50 nm, less than about 20 nm, or even less thanabout 10 nm, and the variation of the film thickness of the surface maybe less than 20%, less than 15%, less than 10%, or less than 5%.

Methods are provided herein to graft this uniform polymeric film onto awide variety of substrate materials. For example, traditionalengineering materials such as stainless steel, titanium, nickel, copper,aluminum, magnesium and/or oxides and/or alloys thereof may be coated bya thin conformal film of polymer to obtain a surface that exhibitsrobust dropwise condensation. According to some embodiments of thepresent invention, semiconductors such as Si, SiC, AN, GaAs, ceramicssuch as TiN, TiC, Sic, SiN, TiO2, and rare-earth oxides can be coated aswell. Methods are also provided to provide a film with controllablethickness and morphology. For example, in certain embodiments, the filmis a conformal film on a textured substrate. In other embodiments, thefilm is a conformal film on a smooth surface. In other embodiments, thefilm is a textured film on a smooth surface.

A film may include or be a thin hydrophobic polymer/copolymer film.Techniques such as initiated chemical vapor deposition (hereafter,“iCVD”) allow deposition of precisely-controlled, extremely thin (e.g.,as thin as 10 nm) polymeric films on metal substrates, where the polymeris covalently bonded to the substrate. Furthermore, the polymeric filmmay be crosslinked at or near its exposed surface and/or throughout thebulk of the film via introduction of a crosslinking agent to the gasstream, and may be followed subsequently by annealing. The resultingfilm or coating exhibits significant reductions in thermal resistance,drop shedding size, and/or degradation rate during dropwise condensationof steam compared to existing coatings. Certain advantages of thedescribed compositions and methods thereof are detailed as follows.

Variability of Film and Substrate Composition

In some embodiments, compositions and methods described herein may havea wide variability of film and substrate materials. Exemplary filmmaterials include, but are not limited to fluoropolymers, includingpoly-tetrafluoroethylene (PTFE), poly-perfluoroacrylates,poly-perfluormethacrylates, and copolymers thereof. Other exemplary filmmaterials include, but are not limited to, poly-methylmethacrylate(PMMA), poly-glycidyl methacrylate (PGMA), and poly-2-hydroxyethylmethacrylate. In certain embodiments, the polymeric film includes afluoropolymer, e.g., PFDA, along with a crosslinker species, e.g.,divinylbenzene (DVB). Some embodiments of the present invention utilizea fluorinated polymer, e.g. PTFE or PFDA, or combination thereof. Forexample, Teflon by DuPont, a PTFE, may be used. Some commercializedfilms of PTFE are available from GVD (http://www.gvdcorp.com/). Suchfilms are described, for example, in U.S. Patent Application PublicationNo. 2013/0280442, U.S. Patent Application Publication No. 2013/0171546,and U.S. Patent Application Publication No. 2012/0003497, although thesefilms as-described would not be suitable for dropwise condensation owingto high contact angle hysteresis and lack of crosslinking or other meansof inducing steric hindrance.

In certain embodiments, the polymeric film includes exemplaryeco-friendly C6-type fluoropolymer materials including, but not limitedto 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl methacrylate, 1H, 1H,2H, 2H-perfluorooctyl acrylate, 2-(perfluorohexyl) ethyl methacrylate,[N-methyl-perfluorohexane-1-sulfonamide] ethyl acrylate,[N-methyl-perfluorohexane-1-sulfonamide] ethyl (meth) acrylate,2-(Perfluoro-3-methylbutyl)ethyl methacrylate, 2-[[[[2-(perfluorohexyl)ethyl] sulfonyl] methyl]-amino] ethyl] acrylate, and copolymers thereof.

In addition, 2-(Perfluoro-3-methylbutyl)ethyl methacrylate (C5PFMA),combined with the crosslinking strategy or graded-structure strategy,can be explored via iCVD polymerization. This monomer has enriched CF₃end groups, which lowers surface energy and promotes hydrophobicity.

In certain embodiments, the polymeric film comprises at least one memberselected from the group consisting of polymethylmethacrylate (PMMA),polyglycidylmethacrylate (PGMA), poly-2-hydroxyethylmethacrylate,polyperfluoroacrylate (PFDA), and copolymers thereof. In certainembodiments, the polymeric film comprises a fluoropolymer. In certainembodiments, the fluoropolymer comprises polytetrafluoroethylene (PTFE).In certain embodiments, the fluoropolymer comprises [C₁₂H₉F₁₃O₂]_(n),where n is an integer greater than zero (e.g.,poly(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl methacrylate), or‘C6’-analog of PFDA). In certain embodiments, the fluoropolymercomprises a copolymer of divinylbenzene (DVB) and one or both of: PFDAand PTFE.

In one aspect, the invention is directed to an article for enhanced heattransfer, the article comprising a substrate and a (e.g., thin, uniform)polymeric film grafted (e.g., covalently bonded) thereupon. In certainembodiments, the substrate comprises a metal (e.g., steel, stainlesssteel, titanium, nickel, copper, and/or alloys thereof). In certainembodiments, the substrate comprises a semiconductor (e.g., SiC, Si,AlN, GaAs, GaN, ZnO, Ge, SiGe, BN, BAs, AlGaAs, TiO₂, etc.). In certainembodiments, the substrate comprises a rare earth element or compoundcomprising a rare earth element (e.g., a rare earth oxide, carbide,nitride, fluoride, or boride; e.g., cerium oxide CeO₂).

In some embodiments, methods are provided herein to graft this uniformpolymeric film onto a wide variety of substrate materials. In certainembodiments, the film is a conformal film on a textured substrate. Forexample, in some embodiments, traditional materials such as stainlesssteel, titanium, nickel, copper, aluminum and/or their alloys may becoated by a thin conformal film of polymer to obtain a surface thatexhibits robust dropwise condensation. According to some embodiments ofthe present invention, semiconductors such SiC, AN, GaAs can be coatedas well.

In some embodiments, the substrate on which the film is depositedincludes plastic, silicon, quartz, woven or non-woven fabric, paper,ceramic, nylon, carbon, polyester, polyurethane, polyanhydride,polyorthoester, polyacrylonitrile, polyphenazine, polyisoprene,synthetic rubber, polytetrafluoroethylene, polyethylene terephthalate,acrylate polymer, chlorinated rubber, fluoropolymer, polyamide resin,vinyl resin, expanded polytetrafluoroethylene, low density polyethylene,high density polyethylene, or polypropylene. In some embodiments, thesubstrate is homogeneous. In some embodiments, the substrate isheterogeneous. In some embodiments, the substrate is planar. In someembodiments, the substrate is non-planar. In some embodiments, thesubstrate is concave. In some embodiments, the substrate is convex. Insome embodiments, the substrate possesses a micro/nanoscale hierarchicaltexture.

Covalent Grafting

In some embodiments, compositions and methods described herein may havea covalently bonded interface between a film and a substrate. Thefilm-substrate interfaces obtained by other methods of deposition, suchas sputtering or casting, suffer from weak bonds between substrate andfilm. When stressed by the large mismatch in coefficient of thermalexpansion (Δα˜1×10⁻⁴), hydrolysis in the presence of steam, or the shearstresses encountered during droplet coalescence, these interfaces havebeen shown to be highly prone to delamination. The covalently bondedinterface used in accordance with some embodiments described herein maybe shown to resist delamination for prolonged periods. The covalentbonding between the film and the substrate can also lower the thermalinterface resistance, thereby improving the overall heat transfercoefficient.

Many different chemistries exist for covalently attaching a vinyl orother reactive group to a substrate, and specifically a metal substrate.Silanes, thiols, carboxylic acids, and phosphonates (or phosphonicacids) are examples of such well-known chemistries. Under someconditions, such as alkaline conditions with pH>7, the hydrolyticstability of phosphonates exceeds that of silanes. Under otherconditions, such as under solar irradiation, silanes are more stablethan phosphonates. Both phosphonates and silanes can possess one or morevinyl functional group. Silanes with more than one anchor point,referred to as dipodal silanes, result in greater stability andsubstrate adhesion.

Tunable Thickness & Morphology

Previous attempts at promoting dropwise condensation, for example withself-assembled monolayers, have usually resulted in films that degradeover time. Monolayers will inevitably have holes in the film that willact as degradation initiation sites. For example, the silane-metal bondsof a silanized substrate are susceptible to hydrolysis by steam. Otherpromoters, such as oleic acids, have been shown to function only oncopper substrates, and are incompatible with the moreindustrially-relevant materials used in heat exchangers such asstainless steel and titanium alloys. A thicker film, e.g., more than amonolayer, will help ensure that there are no regions of exposedsubstrate.

However, since the thermal conductivity of polymers are much lower thanthat of a metal tube (for example, the thermal conductivity of bulk PTFEis approximately 0.25 W/mK as compared to approximately 20 W/mK forstainless steel), previous attempts at obtaining a dropwise promotersurface via polymer films were many microns thick. The additionalthermal resistance posed by such thick films was enough to offset anybenefits of the higher heat transfer coefficient during dropwisecondensation, making these films unusable for promoting dropwisecondensation.

To optimize the film thickness and to ensure that the conductionresistance of the polymer film contributes no more than 1% of the totalresistance, the thickness, in some embodiments, must be less than 1 μm.The total thermal resistance includes the following resistances inseries: the resistance from the condensing vapor to the substrate, theconduction resistance through the film and the substrate, and theconvection resistance of the coolant:

R _(T) =Rs+R _(f) +R _(m) +R _(w)=(1/h_(S))+(1/k)_(f)+(1/k)_(m)+(1/h)_(w)  (1)

where the subscripts s, f, m, and w represent the steam condensation,film conduction, metal conduction, and water convection, respectively.Typical orders of magnitude of the variables are as follows:h_(s)≈10⁴W·m⁻²·K⁻¹, k_(f)≈10⁻¹W·m⁻¹·K⁻¹, l_(f)≈10⁻³m, k_(m)≈10²³W·m⁻²W·m⁻¹·K⁻¹, h_(w)≈10³W·m⁻²·K⁻¹. Thus, the total resistance of thecondenser is on the order of 10⁻³ K·m·W⁻¹, whereas the conductionresistance due to the film is on the order of 10⁻⁸K·m·W⁻¹. Since thepresent coating is so thin (e.g., on the order of 10 nm, 20 nm, 30 nm,40 nm), it represents only about 0.5% of the condensation resistance and˜0.001% of the total thermal resistance. This is in contrast to thepolymer films in conventional systems that were typically many micronsthick.

In some embodiments, a film described herein can be sufficiently thickenough to provide complete coverage, but thin enough to minimize anyadded thermal resistance. The thickness of a film may be preciselycontrolled in real time, for example, by laser interferometry (or othersuitable methods) to obtain films as thin as 10 nm. The thermalresistance of a 10 nm film of PTFE is negligible: 4×10⁻⁸K/W,corresponding to a thermal conductance of 25 MW/m²K.

In certain embodiments, the deposited polymeric film has an averagethickness from 1 nm to 1 micron. In certain embodiments, the depositedpolymeric film has an average thickness from 1 nm to 100 nm.

In certain embodiments, the polymeric film is no greater than 500 nm inthickness (e.g., no greater than 400 nm, no greater than 300 nm, nogreater than 200 nm, no greater than 100 nm, no greater than 75 nm, nogreater than 50 nm, no greater than 25 nm, or no greater than 15 nm,e.g., as thin as 10 nm). In certain embodiments, the polymeric film hasa thickness variation of no greater than about 20% (e.g., no greaterthan about 15%, no greater than about 10%, or no greater than about5%—e.g., the polymer film is uniform). In some embodiments, thethickness of the polymeric film is about 10 nm, about 20 nm, about 30nm, about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 80 nm,about 90 nm, about 100 nm, about 125 nm, about 150 nm, about 175 nm,about 200 nm, about 225 nm, about 250 nm, about 275 nm, about 300 nm,about 325 nm, about 350 nm, about 375 nm, about 400 nm, about 425 nm,about 450 nm, about 475 nm, about 500 nm, about 525 nm, about 550 nm,about 575 nm, about 600 nm, about 625 nm, about 650 nm, about 675 nm,about 700 nm, about 725 nm, about 750 nm, about 775 nm, about 800 nm,about 825 nm, about 850 nm, about 875 nm, about 900 nm, about 1000 nm,about 1100 nm, about 1200 nm, about 1300 nm, about 1400 nm, or about1500 nm.

Minimizing Contact Angle Hysteresis

In certain embodiments, a film is particularly useful for enhancingdropwise condensation. The dropwise heat transfer coefficient isstrongly influenced by the size of the departing drops. Since acondensate drop begins to present a thermal resistance as soon as itforms, it would be desirable to shed condensate drops as soon aspossible. A typical surface will be able to support a drop as it growsto the capillary length, which is approximately 2.7 millimeters forwater. At this size, the drops present a significant thermal barrier. Ifinstead the drops can be shed at a much smaller size, the overall heattransfer coefficient will be increased significantly. External forcessuch as gravity or vapor shear may be utilized to remove condensatedroplets, but they will have to overcome the forces due to surfacetension that pin the contact line of the drop to the condensing surface.A useful measure of the pinning strength of a surface is the contactangle hysteresis (CAH)—the difference between the advancing and recedingcontact angles. A lower CAH will result in easier shedding of condensatedrops. For smooth surfaces, CAH is minimized when the surface is free ofmorphological and chemical inhomogeneities. Thus, a smooth, chemicallyhomogeneous surface is desirable for minimizing CAH and maximizing theheat transfer coefficient.

Additionally, the molecular rearrangement of pendant moieties uponexposure to a wetting fluid such as water gives rise to increased CAH,as explained, for example, in A. Synytska, D. Appelhans, Z. G. Wang, F.Simon, F. Lehmann, M. Stamm, K. Grundke, Macromolecules 2007, 40, 1774.This rearrangement may be prevented by increasing the molecular rigidityvia adjusting the degree of crystallinity and/or the degree ofcrosslinking to minimize the contact angle hysteresis. In someembodiments, compositions and methods described herein may have atunable molecular rigidity. By altering the deposition parameters, themolecular rigidity may be adjusted at any position (e.g., at any depthor location) within the film, including the film-substrate interface andthroughout bulk of the film. At the free surface of the film that isexposed to liquids, it is particularly desirable to obtain rigid films.

In some embodiments, a film is treated (e.g., annealed) afterdeposition. Without being bound to any particular theory, annealing canreduce hysteresis, by increasing the degree of crystallinity and/orincreasing the degree of crosslinking of the film as explained, forexample, in J. L. Yagüe, K. K. Gleason, Macromolecules, 2013, 46, 6548.

For example, a film described according to some embodiments discussedherein may be thermally annealed to improve both durability and contactangle hysteresis (CAH). In experiments described in more detail hereinbelow, copolymer films of poly-perfluorodecyl acrylate anddivinylbenzene (PFDA/DVB) were annealed at 60° C. to improvecrosslinking, resulting in a surface with lower CAH and improveddurability in the presence of high-temperature steam.

The smaller, eco-friendly C₆-type perfluorinated chains are moredifficult to restrain from reorientation upon contact with water, but bycarefully choosing an appropriate spacer group (located between theacrylate backbone and the fluorinated functional group), thisreorientation may be mitigated. For example, a spacer consisting only ofan ethyl group, such as (1H1H2H2HC_(n+2)F_(2n+1)) acrylate,crystallization at room temperature is possible only for n≧8, (forexample such as (1H1H2H2H perfluorodecyl) acrylate with n=8), sinceinteractions between adjacent monomers only occur only between theirperfluorinated pendant groups. However, by substituting a[[sulfonyl]methyl]-amino] spacer for the ethyl spacer as an example,additional dipole-dipole interactions between the spacer groups ofadjacent monomers are able to further restrain pendant groups andpromote crystallization of smaller perfluorinated chains. Referring nowto FIG. 23, we find that films of[N-methyl-perfluorohexane-1-sulfonamide] ethyl (meth) acrylate (C6PFSMA)exhibit significantly smaller contact angle hysteresis compared to filmsof [N-methyl-perfluorohexane-1-sulfonamide] ethyl acrylate (C6PFSA) andPoly(2-(Perfluorohexyl)ethyl methacrylate) (pC6PFMA).

A polymeric film may be crosslinked to improve rigidity and minimizeCAH. Exemplary crosslinkers include, but are not limited to,divinylbenzene (DVB), ethylene dimethyacrylate (EDMA),di(ethyleneglycol) di(methacrylate), di(ethyleneglycol) di(acrylate),ethyleneglycoldimethacrylate (EGDMA) and di(ethyleneglycol)di(vinylether) (DEGDVE), and/or 1H, 1H, 6H, 6H-perfluorohexyldiacrylate.

The contact angle hysteresis of iCVD films with various liquids is givenin Table 1 below. The contact angle hysteresis may be measured with agoniometer by injecting liquid into a drop to measure the advancingcontact angle, and withdrawing liquid from the drop to measure thereceding contact angle.

TABLE 1 Contact angle hysteresis of iCVD films described herein withvarious liquids iCVD film Liquid Δθ [°] PFDA homopolymer Water 5 PFDAhomopolymer Mineral oil 22 PFDA-co-DVB copolymer Hexane 11 PFDA-co-DVBcopolymer Pentane 8

In certain embodiments, the polymeric film has a surface (e.g., exposedsurface) with low contact angle hysteresis (e.g., no greater than 50°,no greater than 40°, no greater than 30°, no greater than 25°, nogreater than 20°, no greater than 15°, or no greater than 10° for water,where contact angle hysteresis is the difference between advancingcontact angle and receding contact angle). In certain embodiments, thepolymeric film has a surface (e.g., exposed surface) with high advancingcontact angle (e.g., no less than 70°, no less than 80°, no less than90°, no less than 100°, no less than 120°, no less than 130° for water)and/or high receding contact angle (e.g., no less than 60°, no less than70°, no less than 80°, no less than 90°, no less than 100°, no less than110°, or no less than 120° for water). In some embodiments, theadvancing water contact angle is greater than about 150°. In someembodiments, the advancing water contact angle is about 150°, about155°, about 160°, about 165°, or about 170°. In some embodiments, thereceding water contact angle is greater than about 150°. In someembodiments, the receding water contact angle is about 150°, about 155°,about 160°, about 165°, or about 170°.

Preferably, the contact angle hysteresis is <25°. More preferably, thecontact angle hysteresis is <5°. If the contact angle hysteresis ishigher, it may be compensated for by a lower surface energy, which wouldresult in a larger contact angle and a larger gravitational body forceper length of contact line acting to shed the drop.

In some embodiments, the water contact angle hysteresis is about 10°,about 9°, about 8°, about 7°, about 6°, about 5°, about 4°, or about 3°.In some embodiments, the water contact angle hysteresis is between about3° and about 10°.

In some embodiments, the advancing mineral oil contact angle is greaterthan about 100°. In some embodiments, the advancing mineral oil contactangle is about 100°, about 105°, about 110°, about 115°, about 120°,about 125°, or about 130°. In some embodiments, the advancing mineraloil contact angle is between about 100° and about 130°.

In some embodiments, the receding mineral oil contact angle is greaterthan about 100°. In some embodiments, the receding mineral oil contactangle is about 100°, about 105°, about 110°, about 115°, about 120°,about 125°, or about 130°. In some embodiments, the receding mineral oilcontact angle is between about 100° and about 130°.

In some embodiments, the static mineral oil contact angle is greaterthan about 100°. In some embodiments, the static mineral oil contactangle is about 100°, about 105°, about 110°, or about 115°. In someembodiments, the static mineral oil contact angle is between about 100°and about 115°.

iCVD Coating Process

Coating typically involves the deposition of films or layers on asurface of a substrate. One manner of effecting the deposition of suchfilms or layers is through chemical vapor deposition (CVD). CVD involvesa chemical reaction of vapor phase chemicals or reactants that containthe constituents to be deposited on the substrate. Reactant gases areintroduced into a reaction chamber or reactor, and are decomposed andreacted at a heated surface to form the desired film or layer.

In some embodiments, CVD used in accordance with the present inventionis an initiated CVD (iCVD). iCVD Deposition Example in CylindricalReactor below discusses a typical experimental set-up for iCVD. In aniCVD process, thin filament wires are heated, thus supplying the energyto fragment a thermally-labile initiator, thereby forming a radical atmoderate temperatures. The use of an initiator not only allows thechemistry to be controlled, but also accelerates film growth andprovides control of molecular weight and rate. The energy input is lowdue to the low filament temperatures, but high growth rates may beachieved. The process progresses independent of the shape or compositionof the substrate, is easily scalable, and easily integrated with otherprocesses.

In certain embodiments, iCVD takes place in a reactor. In certainembodiments, a variety of monomer species may be polymerized anddeposited by iCVD. In certain embodiments, the surface to be coated isplaced on a stage in the reactor and gaseous precursor molecules are fedinto the reactor; the stage may be the bottom of the reactor and not aseparate entity. In certain embodiments, a variety of carrier gases areuseful in iCVD.

In certain embodiments, the iCVD reactor has automated electronics tocontrol reactor pressure and to control reactant flow rates. In certainembodiments, unreacted vapors may be exhausted from the system.

The iCVD process is a single-step, solvent-free, low-energy, vapor-phasemethod used to deposit conformal films with precisely controllablethickness and in which grafting to the substrate provides enhanceddurability, as discussed, for example, in M. E. Alf, A. Asatekin, M. C.Barr, S. H. Baxamusa, H. Chelawat, G. Ozaydin-Ince, C. D. Petruczok, R.Sreenivasan, W. E. Tenhaeff, N. J. Trujillo, S. Vaddiraju, J. Xu, K. K.Gleason, Adv. Mater. 2010, 22, 1993. The large choice of suitablemonomers that may be used allows for precise design and modulation ofsurface properties.

Certain embodiments presented herein relate to films exhibiting acombination of durability and low contact angle hysteresis.Copolymerization with a crosslinker is an additional method that aids inboth further reduction of contact angle hysteresis and also renderingthe films more stable to chemical and mechanical degradation—making thefilms more robust and extending the useful life of those films.

Certain embodiments presented herein relate to the use of vaporsynthesis for copolymerization, which in some embodiments does notrequire that the two monomers being copolymerized have a common solvent.This characteristic will be recognized by those skilled in the art as asignificant advantage over wet-chemistry synthesis techniques, as acommon solvent does not exist for PFDA and DVB. In some embodiments,iCVD allows a non-fluorinated crosslinker, DVB, to be readilycopolymerized with the fluorinated monomer, PFDA, over its entirecompositional range.

Copolymerization also disrupts crystallization. Since crystallites areone source of roughness, copolymer films in some embodiments may be madeto be smoother than crystalline iCVD p(PFDA) homopolymer layers. Suchsmooth surfaces may be desired to reduce the contact angle hysteresis oflow-surface tension fluids such as hydrocarbons, refrigerants, and/orcryogens. Additionally, the perfluorinated side chains of the PFDA unitssegregate to the interface under dry conditions in order to minimizesurface energy. Surface reconstruction in which the perfluoro chainsorient away from the interface can occur when the surface becomes wet.

The iCVD of homopolymers p(PFDA) and p(DVB) results in highly conformalthin films, and superhydrophobic and superoleophobic surfaces have beendemonstrated with iCVD films of p(PFDA).

Certain embodiments described herein prevent the reorientation of CF₃groups via crosslinking. A crosslinking agent provides a controllablemeans of steric hindrance, because the proportion (e.g., concentrationin particular location) of crosslinking agent may be varied along thefilm thickness. FIG. 18 schematically illustrates embodiments employingvariation in degree of crosslinking and/or variation in concentration ofcrosslinking agent as a function of position of the crosslinking agentalong the thickness of the polymeric film. The polymeric film includes agrafting layer (e.g., where the grafting layer has a thickness fromabout 0.5 nm to about 5 nm, or from about 1 nm to about 3 nm, or fromabout 1 nm to about 2 nm), and a bulk film layer making up the majority(e.g., more than 50%, more than 55%, more than 60%, more than 70%, morethan 80%, more than 90%, more than 95%) of the polymeric film. Incertain embodiments, the polymeric film has at thickness no greater than400 nm, no greater than 300 nm, no greater than 200 nm, no greater than100 nm, no greater than 75 nm, no greater than 50 nm, no greater than 25nm, or no greater than 15 nm, in thickness. In some embodiments, thepolymeric film may be as thin as 10 nm or have a thickness on the orderof 10 nm.

One of the main difficulties in obtaining a surface that exhibitsdropwise condensation of hydrocarbons and other low-surface tensionliquids has been obtaining a surface with a sufficiently low criticalsurface tension. The condensate will spread to form a film unless thecritical surface tension of the surface is below that of the condensingliquid. Table 2 below lists the surface tension values for water and avariety of other liquids, including n-alkanes (octane, hexane, pentane)and a fluorocarbon similar to a typical refrigerant. Table 3 listsrefrigerants, e.g., hydrofluorocarbons, chlorofluorocarbons, andhydrochlorofluorocarbons.

The n-alkanes have surface tensions that are considerably lower thanwater, and also lower than most common industrial materials (includingpolymers) whose critical surface tensions are shown in Table 4. Forexample, Teflon has a surface energy of 19 mN/m, since it is composedprincipally of CF₂ groups, and is not sufficient to condense hexane orlower alkanes. Even trichloro(1H,1H,2H,2H-perfluorooctyl)silane(commonly referred to as fluorosilane, a low-surface energy fluorinatedsilane surface modifier), has a critical surface energy of 10 mN/m.Although fluorosilane is terminated by CF₃ groups, the lack ofcrosslinking or other steric hindrance allows these CF₃ group toreorient in the presence of water or another wetting liquid. As aresult, it is difficult to obtain a surface with a critical surfaceenergy low enough to promote dropwise condensation of these liquids.

TABLE 2 Surface tensions of water and various low-surface tensionfluids. σ_(iv) @25° C. liquid [mN/m] water 72.71 toluene 27.93isopropanol 20.92 ethanol 24.77 octane 21.08 hexane 17.98 pentane 15.47perfluorohexane 11.47

TABLE 3 List of refrigerants. Chlorofluorocarbons R-11Trichlorofluoromethane R-12 Dichlorodifluoromethane R-13Chlorotrifluoromethane R-13B1 Bromotrifluoromethane R-14Tetrafluoromethane R-113 Trichlorotrifluoroethane R-1141,2-Dichloro-1,1,2,2-Tetrafluoroethane R-500 Dichlorodifluoromethane,Difluoroethane R-502 Chlorodifluoromethane, ChloropentafluoroethaneR-503 Chlorotrifluoromethane, Trifluoromethane HydrochlorofluorocarbonsR-12 1-Chloro-1,2,2,2-tetrafluoroethane, 1,1,1,2-Tetrafluoroethane R-22Chlorodifluoromethane R-123 Dichlorotrifluoroethane R-1241-Chloro-1,2,2,2-Tetrafluoroethane R-401A Chlorodifluoromethane,Chlorotetrafluoroethane R-401B Chlorodifluoromethane,Chlorotetrafluoroethane R-402A Chlorodifluoromethane, PentafluoroethaneR-402B Chlorodifluoromethane, Pentafluoroethane R-408A Trifluoroethane,Chlorodifluoromethane R-409A Chlorodifluoromethane,Chlorotetrafluoroethane R-412A Chlorodifluoromethane,1-Chloro-1,1-Difluoroethane and Octafluoropropane R-414BChlorodifluoromethane, Chlorodifluoroethane, ChlorotetrafluoroethaneR-416A 1-Chloro-1,2,2,2-tetrafluoroethane, 1,1,1,2-TetrafluoroethaneHydrofluorocarbons R-23 Trifluoromethane R-116 Hexafluoroethane R-134a1,1,1,2-Tetrafluoroethane R-404A Pentafluoroethane,1,1,1,2-Tetrafluoroethane, Trifluoroethane R-407A Difluoromethane,Pentafluoroethane, 1,1,1,2-Tetrafluoroethane R-407B Difluoromethane,Pentafluoroethane, 1,1,1,2-Tetrafluoroethane R-407C Difluoromethane,Pentafluoroethane, 1,1,1,2-Tetrafluoroethane R-410A Pentafluoroethane,Difluoromethane R-417A 1,1,1,2-Tetrafluoroethane and PentafluoroethaneR-422A 1,1,1,2-Tetrafluoroethane and Pentafluoroethane R-422D1,1,1,2-Tetrafluoroethane and Pentafluoroethane R-423ATetrafluoroethane, Heptafluoropropane R-427A 1,1,1,2-Tetrafluoroethane,Pentafluoroethane R-438A Difluoromethane, Pentafluoroethane, 1,1,1, 2-Tetrafluoroethane, n-Butane, Isopentane R-507 Pentafluoroethane,Trifluoroethane R-508A Trifluoromethane, Hexafluoroethane R-508BTrifluoromethane, Hexafluoroethane

TABLE 4 Critical surface energy of industrial polymers. Surface ContactPolymer Energy Angles Abbr. Polymer Name (dynes/cm) (degrees) PESPolyethersulfone 46 90 Styrene butadiene rubber 48 PPO Polyphenyleneoxide 47 75 Nylon 6/6 (polyhexamethylene adipamide) 46 PC Polycarbonate46 75 Nylon-6 (polycaprolactam) 38 PET Polyethylene terephthalate 42 76PMMA Polymethylmethacrylate 41 82 SAN Styrene acrylonitrile 40 74Polyimide 40 83 PCV r Polyvinyl chloride, rigid 39 90 Polyester 41 70Acetal 36 85 ABS Acrylonitrile butadiene styrene 35 82 PPS Polyphenylenesulfide 38 87 PVA Polyvinyl alcohol 37 10 Polyacrylate (acrylic film) 35PVC p Polyvinyl chloride, plasticized 35 89 PS Polystyrene 34 72Nylon-12 36 Surlyn ionomer 33 80 PBT Polybutylene terephthalate 32 88CTFE Polychlorotrifluoroethylene 31 PP Polypropylene 30 88 PUPolyurethane 38 85 PE Polyethylene 30 88 PVF Polyvinyl fluoride 28 PVDFPolyvinylidene fluoride 25 80 Natural rubber 24 PDMS Polydimethylsioloxane (silicone elastomer) 23 98 FEP Fluorinated ethylene propylene20 98 PTFE Polytetrafluoroethylene 19 120

Even if a surface can be found with sufficiently low surface energy toavoid spreading of the condensate, a second difficulty in obtainingdropwise condensation of low surface tension liquids is reducing thecontact angle hysteresis (and thus the drop adhesion). If the adhesionof the condensate drops to the surface is high, then the drops will beunable to shed from the surface, and the initial dropwise condensationwill proceed until the individual drops merge to form a continuous film.This is an especially difficult problem in the case of low-surfacetension fluids. Since the contact angle of a condensate drop willinevitably be low (in the range of about 10° to 30°), the ratio of thebody force due to gravity acting to shed the drop will be small comparedto the force acting to pin the drop to the surface. A plot showing theeffect of contact angle θ on heat transfer coefficient h is shown inFIG. 19, where maximum h for water is at 0˜90° and for octane ˜50°.

Surfaces that promote dropwise shedding of low-surface tensioncondensates, such as liquid hydrocarbons, are demonstrated in theexperimental examples presented herein. For example, demonstrated hereinis the dropwise condensation of hexane on a surface comprising iCVDcopolymer of PFDA-co-DVB. FIGS. 22A and 22B are photographic stills froma video showing dropwise condensation and shedding of n-hexane on aPFDA-co-DVB on silicon substrate, where P=15 kPa, T_(s)=10±1° C.,T_(sat)=18.3° C., and ΔT=8.3±1° C.

Surfaces such as the ones shown in FIGS. 20a and 20b have valuableapplications in a wide variety of industries, for example, inapplications of refrigeration, dehumidification, and HVAC, whichcondense a refrigerant, generally a low-surface tension fluorocarbonfluid. Condensers that promote dropwise shedding of such fluids resultin higher overall efficiencies and/or lower device footprint. Furtherapplications include power plants utilizing organic Rankine cycles,e.g., with isobutene, pentane, or propane as the working fluid, whichmay allow for smaller condensers to be used, and lower capital costs forsuch power plants. Other applications include the fractionation ofhydrocarbon crude streams into constituent components, allowing forsmaller fractionation columns with fewer stacks.

Also presented herein is the finding that surfaces with both (1) lowcritical surface energy and (2) low contact angle hysteresis promotedropwise shedding of low-surface tension condensates such as liquidhydrocarbons. Furthermore, owing to the grafting (e.g., covalentbonding) of the film to the substrate, these surfaces display a highdegree of robustness. They are seen to survive prolonged condensation in100° C. steam with no noticeable degradation.

For example, the critical surface energy of an iCVD-grafted PFDAhomopolymer has been determined to be 5.6 mN/m, as compared to 18.5 mN/mfor the ungrafted homopolymer. Critical surface energy may be determinedby plotting 1-cos(θ_(a)), where θ_(a) is the cosine of the advancingcontact angle of a homologous series of liquids, e.g. n-heptane,n-octane, n-decane, etc., and finding the x-intercept. In certainembodiments, the critical surface energy is <18 mN/m. If the criticalsurface energy is higher, the surface may become flooded by thelow-surface tension fluid. In some embodiments, the critical surfaceenergy is <6 mN/m.

There is a wide array of industrial applications for iCVD coatings fordropwise condensation and shedding of low-surface tension liquids.Condensation of low-surface tension liquids in industrial applicationshas occurred in the filmwise mode due to the aforementioned difficultiesin achieving dropwise condensation and shedding. As a result, since thethermal conductivities of low-surface tension fluids (typically <0.2W/mK) are worse even than water (0.6 W/mK), these condensers suffer fromconsiderable thermal inefficiencies due to the thermal resistance of thecondensate film. By implementing a dropwise condenser, e.g., with iCVDcoating as described herein, the heat transfer coefficient [W/m²K] canincrease tenfold. Thus, in certain embodiments, for a given coolanttemperature, ten times the heat can be transferred, or the same amountof heat can be transferred by a heat exchanger that is smaller than theoriginal size or by a temperature difference that is smaller than theoriginal temperature difference.

The coatings/surfaces described herein have numerous important uses inoil and gas processing (e.g., LNG, propane, etc.); refrigerants,condenser coils in dehumidification systems, commercial/residentialHVAC, consumer packaging, medical devices, water recovery from coolingtowers, dew/fog collection, organic Rankine cycles, steam based powergeneration (e.g., solar thermal, geothermal, etc.), liquefaction(including LNG, CO₂, N₂, liquid oxygen, etc.), and phase transitionapplications involving mitigation of icing, hydrates, and scaleformation.

In applications of refrigeration, dehumidification, and HVAC whichcondense a refrigerant (typically a low-surface tension fluorocarbonfluid), dropwise condensers would result in higher overall efficienciesand lower device footprint. In power applications utilizing organicRankine cycles, e.g. with isobutene, pentane, or propane as the workingfluid, condensers must be used to pull the working fluid through theturbine and condense back to liquid to be pumped back through the cycle.Implementing dropwise condensers would allow for smaller equipment size,which would significantly reduce the capital cost of such plants; andwould also allow for better overall cycle efficiencies. In thefractionation of crude streams into constituent components, e.g.kerosene, alkanes, fuel oils, and diesel and heavier fuels, dropwisecondensing surfaces would allow for smaller fractionation columns withfewer stacks. In applications such as the liquefaction of natural gas,oxygen, and nitrogen, cold boxes are used to condense a gas stream intoa liquid. The cooling flux of the cold box is provided by a portion ofthe liquefied product, and so by increasing the heat transfercoefficient of the condensers, the liquefaction plant would be able toproduce a larger amount of valuable liquid product instead ofless-valuable gaseous product. Furthermore, with the advent ofship-based liquefaction plants, the heat transfer equipment becomesseverely space-constrained. A dropwise condenser would provide the sameheat flux in a much smaller footprint than the current filmwisecondensers.

Industrial applications of the surfaces described herein include phasechange applications, wherein the surfaces minimize adhesion of solidphases nucleating and growing on the surfaces, e.g., where there is iceformation on power lines, wind turbines, aircraft, and municipalpipelines; where there is hydrate formation on oil and gas equipment(e.g., undersea); and where there is scale formation on equipment inpower plants and boilers, in desalination plants, and municipalpipelines. The low hysteresis of the coatings/surfaces described hereincan be exploited for shedding (e.g., dropwise shedding) of unwantedliquid drops, as in water from car windshields, solar panels, andindustrial machinery; oil contaminants from household cookware, consumerelectronics, and industrial machinery; and blood and other biologicalfluids from medical devices. The low surface energy of thecoating/surfaces described herein can also be exploited for their lowsolid-solid frictional properties, e.g., sliding linear bearings,bushings, and non-stick household implements.

In certain embodiments, a film described herein is used in power plants,desalination condensers, humidification-dehumidification systems, orheating, ventilation, and air conditioning (HVAC). In certainembodiments, a film is used in a thermal interface material (TIM)because of its covalent bonding and flexibility. In certain embodiments,a film is used for cooling of electronics and photonics.

In certain embodiments, the surface energy of a thin film (e.g., a filmof fluorinated polymer) is sufficiently low to be oleophobic, whichwould allow it to be used for dropwise condensation of hydrocarbons.

Some examples below discuss sustained dropwise condensation of steam ona thin film of poly-(1H,1H,2H,2H-perfluorodecyl acrylate)-co-divinylbenzene (p(PFDA-co-DVB).

It is found that roughness can be precisely specified and designed sothat it is high enough to enhance nucleation density but low enough suchthat it does not adversely affect hysteresis. Roughness may be designedby numerous methods, including, for example, degree of crystallization,extent of crosslinking, composition of crosslinker, and substratetemperature during deposition.

Also described herein are findings regarding variables of the describedcoatings/surfaces, including surface energy, roughness, and substratebonding.

Regarding surface energy, it is found that the surface energy of thesurface/coating should be lower than the condensate liquid. For example,for the PFDA-co-DVB copolymer described herein, surface energy may bedetermined from a ratio of CF₃ groups to CF₂ groups at the surface,where σ_(CF3)=6 mN/m and σ_(CF2)=18 mN/m. It is found that on anon-crosslinked surface (e.g., fluorosilane), CF₃ groups re-orient awayfrom the surface when exposed to water. It is also found that DVBcrosslinking rigidifies the CF₃ groups of the PFDA and preventsreorientation. Furthermore, it is found that grafting forces orientationof CF₃ groups toward the surface.

Regarding roughness, it is found that roughness should be low enough toavoid contact angle hysteresis. For example, roughness features smallthan ˜100 nm are “weak” defects and do not contribute to hysteresis. Itis also found that some small amount of roughness is beneficial forproviding nucleation sites. Moreover, roughness can be controlled bycrosslinking. For example, PFDA homopolymer (non-crosslinked)crystallizes into large hemispherical agglomerations. Crosslinkingprevents crystallization and lowers roughness. Copolymer films ofp(PFDA-co-DVB) exhibit a much smaller degree of crystallinity than PFDAhomopolymer, however still exhibit semicrystalline agglomerations thatenhance the nucleation density.

Regarding substrate bonding, it is found that covalent bonds of thepresent coatings/surfaces are stronger than van der Waals bonds oftypical Teflon coatings. Moreover, the vinyl group of PFDA is found tobond covalently with an initiated vinyl group on the surface.

Experimental Examples iCVD Deposition Experiment A—p(PFDA-co-DVB)

In this Example, polymerizations were conducted in a custom-designcylindrical reactor (diameter 24.6 cm and height 3.8 cm). On top of thereactor laid a quartz top that allowed laser interferometry (633-nmHe—Ne laser, JDS Uniphase) for in-situ film thickness monitoring. Insidethe reactor, 14 parallel ChromAlloy filaments (Goodfellow) wereresistively heated at 230° C. and the stage was back-cooled at aconstant temperature of 30° C. by water using a recirculatingchiller/heater (Neslab RTE-7). Reactor pressure was maintained at 200mTorr using a throttle valve (MKS Instruments). The radical initiator,and the gas carrier were delivered inside the reactor through mass flowcontrollers (MKS Instruments). The fluorinated PFDA monomer and the DVBcross-linker were heated in a glass jar to a temperature of 80° C. and60° C. respectively, and their flows were controlled by needle valves.The flow rates of initiator and monomer were kept constant at 3.2 and0.2 sccm. For the different experiments, the flow rate of crosslinkerwas varied to 0, 0.2, 0.6 and 1 sccm, and a patch flow of gas carrierwas introduced to keep a total flow of 5 sccm. Thickness samples rangedfrom 10 nm to 3 μm. FIG. 10 shows incorporation of DVB in the copolymerfilm.

iCVD Deposition Experiment B—Grafted p(PFDA-Co-DVB)

To deposit a silane adhesion layer prior to grafted iCVD polymerization,substrates were first cleaned by sonication in acetone for 5 minutes,followed by rinsing in DI water (18 MOhm), followed by sonication inisopropanol for 5 minutes, and finally a rinse with DI water. Thesurfaces were treated with oxygen plasma for 10 minutes for furthercleaning and for creating surface hydroxyl groups. After plasmatreatment, the surfaces were immediately placed in a vacuum desiccatoralong a small open vial containing 100 μL of either trichlorovinylsilane(97%, Sigma Aldrich) as a grafting precursor for the polymer films. Thechamber was pumped down to 200 mTorr, and the chamber was isolated toallow the silane to vaporize. The chamber was purged twice more, thenisolated. The silane was allowed to vaporize and react with thesubstrate for 2 hours. After deposition, the surfaces were sonicated intoluene to remove excess unreacted silane and rinsed with DI water.

iCVD polymerizations were conducted in a custom-design cylindricalreactor (diameter 24.6 cm and height 3.8 cm), supporting an array of 14parallel chromoalloy filaments (Goodfellow) suspended 2 cm from thestage. Tert-butyl peroxide (TBPO) (98%, Aldrich), PFDA (97%, Aldrich),and DVB (80%, Aldrich) were used as received. The peroxide initiator,TBPO, was delivered into the reactor through a mass flow controller (MKSInstruments) at a constant flow rate of 3.2 sccm. PFDA and DVB werevaporized in glass jars that were heated to 80 and 60° C., respectively.The flow rates were controlled using needle valves and kept constant at0.2 and 0.6 sccm. The filaments were resistively heated to 230° C. usinga DC power supply (Sorensen), and the temperature was measured by aK-type thermocouple (Omega Engineering). The sample stage was backcooledat 30° C. using a recirculating chiller/heater (Neslab RTE-7). Theworking pressure was maintained at 200 mTorr using a throttle valve (MKSInstruments). The reactor was covered with a quartz top (2.5 cm) thatallowed for in-situ thickness monitoring by interferometry with a 633 nmHeNe laser source (JDS Uniphase). Final thickness of the copolymerdeposited on the metal substrate corresponded to 40 nm. Afterwards, athermal annealing process was performed by introducing the sample in anoven (VWR) at 80° C. for 30 min. The full width at half-maximum (FWHM)was fixed at 2-3 eV to take into account the broadening of the 1 eVelectron beam, while using XPS Scienta Database F1s peaks with FWHM of 2eV.

iCVD Deposition Experiment C—Annealing

This Example characterizes samples that were prepared via iCVDdeposition of p(PFDA-co-DVB) on silicon substrates before and afterannealing. iCVD films were prepared in the same manner described in iCVDDeposition Example—p(PFDA-co-DVB, and then further characterized by AFM.After iCVD deposition, samples were annealed in a furnace at 80° C. for30 min and characterized again by AFM. Referring now to FIG. 9, weobserve that after annealing, the quadratic mean roughness of allsurfaces decreases, indicating an increase in the degree ofcrystallinity in the case of the PFDA homopolymer and an increase in thedegree of crosslinking in the cases of the DVB-crosslinked copolymers.Referring now to FIG. 17 showing a comparison of XRD spectra of PFDAhomopolymer and p(PFDA-co-DVB) films before and after thermal annealing,we also observe an increase in the degree of crystallinity of PFDAhomopolymer as evidenced by the increased area under the curvecorresponding to intensity vs. 20, and a decrease in degree ofcrystallinity of the crosslinked polymers as evidenced by a decrease inthe area under the curve corresponding to intensity vs. 20.

iCVD Deposition Experiment D—Eco-Friendly pC6PFA-Co-DVB

In this Example, films of varying compositional ranges of1H,1H,2H,2H-perfluorooctyl acrylate) (pC6PFA; C6) and divinylbenzene(DVB) were deposited via iCVD on silicon wafer substrates. Flowrates ofmonomer and initiator species and nitrogen patch flow are indicated inTable 5 below.

TABLE 5 Nomenclature and flow rates of precursors Flow rate (sccm)Sample C6PFA DVB TBPO N₂ C0 0.2 0 1.2 1.6 C1 0.2 0.2 1.2 1.4 C2 0.2 0.41.2 1.2 C3 0.2 0.6 1.2 1 C4 0.2 1 1.2 0.6

The Fourier transform infrared spectroscopy (FT-IR) spectra of the filmsare shown in FIG. 21. pC6PFA homopolymer gives a sharp band due tocarbonyl group at 1743 cm⁻¹. The two bands at 1237 and 1204 cm⁻¹ arecaused by the asymmetric and symmetric stretching of the —CF₂— moiety,respectively. The sharp band at 1146 cm⁻¹ is caused by the —CF₂—CF₃ endgroup. The pDVB homopolymer FT-IR spectrum shows the —CH₂— stretchingbands at 2871 cm⁻¹, confirming the formation of backbone. The aromatic—CH— contribute to bands between 3000 and 3100 cm⁻¹. The bands between700 and 1000 cm⁻¹ are characteristics of substituted phenyl groups. Theband at 903 cm⁻¹ results from unreacted vinyl groups. The copolymerpresents all the characteristic bands associated with its components.The FT-IR results show the incorporation of the two monomers into thecopolymer film and the retention of the chemical functionality from bothreactants after the polymerization.

The effects of DVB crosslinking on CAH were studied by WCA measurements(FIG. 22). pC6PFA homopolymer film presents high static WCA andadvancing WCA, but low receding WCA. This behavior of pC6PFA surface hasbeen well explained: in its dry state, the fluoroalkyl side chainsorient to the outermost surface layer due to phase segregation betweenhydrogenated and fluorinated moieties. Surface reorganization occurs inpresence of water, leading to surface exposure of hydrophilic moieties.The econstruction easily happens because pC6PFA is unable to formcrystalline structure. In contrast, p(C6PFA-co-DVB) films show improveddynamic water repellency. The receding WCA of all copolymer films aresignificantly enhanced. The movement of water front can be affected bysurface roughness, heterogeneity, reorientation, and mobility. The AFMobservation of films shows that the differences in roughness are notsignificant enough to influence the WCA hysteresis. Therefore theresults suggest that the crosslinking of DVB units hinders thereorientation of surface fluorine groups. It is hypothesized here thatthe DVB units have two effects, on main chain and side chainrespectively, contributing to the restrain of fluorine groups (Fig. X):first, the rigid crosslinker can reduce the flexibility of main chain,reducing the T_(g); second, the planar crosslinker can stericallymitigate side chain reconstruction by reducing free volume.

Film Deposition Experiment—Effect of Spacer Groups

Do demonstrate the ability of the spacer group to affect the rigidityand thus CAH of deposited films, thin films of1H,1H,2H,2H-perfluorooctyl acrylate (C6PFA)Poly(2-(Perfluorohexyl)ethylmethacrylate) (pC6PFMA) [N-methyl-perfluorohexane-1-sulfonamide] ethylacrylate (C6PFSA) and [N-methyl-perfluorohexane-1-sulfonamide] ethyl(meth) acrylate (C6PFSMA) were spin-coated onto silicon substrates.Advancing and receding contact angles and CAH are shown in FIG. 24,indicating that the additional dipole-dipole interactions afforded bythe spacer group of pC6PFSMA act to significantly reduce the CAH ascompared to pC6PFA as shown in FIG. 23.

Film Characterization Experiment a—XPS Spectra

FIG. 5 (left) shows the high-resolution C1s X-ray photoelectron spectra(XPS) of the iCVD p(PFDA-co-DVB) copolymer surface. The pendant groupsfrom the PFDA consist of —CF₂— and —CF₃— and these two bondingenvironments can be readily resolved at 290.8 and 293.1 eV,respectively. In aggregate, these fluorinated carbon groups account for61.8±0.4% of the area of the spectrum. The assignments at lower bindingenergies represent carbon items directly bonded only to oxygen,hydrogen, or other carbon atoms. However, the precise assignments of thepeaks at lower binding energy is ambiguous due to the multitude ofenvironments arising from the main acrylate portion of the PFDA and fromthe DVB.

The —CF₂— and —CF₃— bonding environments were previously observed in C1sXPS spectrum of the iCVD PFDA homopolymer, representing a combined areaof 61.4±0.3% and in agreement with the structural formula for PFDA whichgives a theoretical value of 61.5%. The similarity with homopolymerresults suggests the degree of DVB crosslinker incorporation in thecopolymer in the near-surface region probed by XPS is quite low. Thus,the surface properties of the copolymer in the dry state, such as theadvancing contact angle, will be dominated by the PFDA units. Whenexamined by Fourier transform infrared spectra (FTIR), which penetratesthe entire film thickness, sp²C—H stretching modes between 2810 and 2890cm⁻¹ were observed, confirming the incorporation of the DVB in the bulkof the film. These underlying crosslinking units are anticipated toreduce the ability of the surface layer to reconstruct between the dryand wet states, potentially reducing this contribution to contact anglehysteresis. By following a deposition of PFDA:DVB 0.2:0.6 sccm with athermal annealing step, the advancing and receding water contact angleson the resultant thin film are 132°±1° and 127°±1°, respectively, with aCAH of 5°. Average film thicknesses were measured by ellipsometry, AFM,and contact profilometry to be 41.5±2.4 nm. AFM scans (FIGS. 1d and 1e )illustrate that the surface is covered by structures with a height ofca. 100 nm and an average spacing of 1.3±0.7 μm, resulting in an RMSroughness of 75 nm. These rough features are semicrystalline aggregatesformed at nucleation sites during the condensation polymerizationreaction of the monomers.

Previous literature has shown that —(CF₂)—CF₃ chains with n≧leads toaggregates in a smectic B structure that arrange into a rotationallysymmetric fiber texture. On the other hand, the fluorosilane surface,which is composed of larger, less sterically-hindered functional groupswith a thickness of 2.5 nm and an RMS roughness of 1.5±0.3 nm, exhibiteda CAH of 25°±3°. Since the roughness of the silanized surface is lowerthan that of the copolymer surface, morphology alone cannot explain thelower hysteresis of the copolymer surface. Instead, this may beattributed to the steric hindrance induced by the crosslinking thatprevents the —CF₃ groups from shifting away from their low-energyunwetted state.

Film Characterization Experiment B—Film Thickness Measurements

Film thicknesses were measured with variable-angle ellipsometricspectroscopy (VASE, M-2000, J. A. Woollam) and by measuring scratch stepheight with atomic force microscopy (AFM, MP3D-SA, Asylum) and contactprofilometry (Model 150, Dektak). All VASE thickness measurements wereperformed at a 70° incidence angle using 190 different wavelengths from315 to 718 nm. A nonlinear least-squares minimization was used to fitellipsometric data of dry films to the Cauchy-Urbach model. Thethickness was obtained upon convergence of the algorithm. FTIRmeasurements were performed on a Nicolet Nexus 870 ESP spectrometer innormal transmission mode equipped with a MCT (mercury cadmium telluriumdetector and KBr beamsplitter. Spectra were acquired over the range of400 to 4000 cm⁻¹ with a 4 cm⁻¹ resolution for 256 scans. All AFMthickness measurements were performed in tapping mode over an area of 20μm×20 μm using a cantilever with a tip radius of 9±2 nm (AC200TS,Asylum). The film thickness was calculated as the difference between theaverage heights of the rough film surface and the trough of the scratch;the rough built-up edge of the scratch was masked from analysis. Theprofilometry measurements were performed with a stylus having a radiusof 12.5 μm. The film thickness was similarly calculated as thedifference in the average height of the rough film and the smoothscratch trough. AFM and profilometry measurements were repeated on atleast four locations. Film thickness is reported as the mean andstandard deviation of all measurements.

Film Characterization Experiment C—Surface Roughness Measurements

Surface roughness was measured using atomic force microscopy (AFM,MP3D-SA, Asylum) in tapping mode. The advancing and receding contactangles were measured using a goniometer (Model 590 Advanced, ramé-hart).The hysteresis was also measured during condensation on the graftedpolymer sample as the difference between the receding and advancing endsof a drop immediately before departure. Contact angles duringcondensation on the silanized sample could not be measured due to thefilm covering the surface.

Dropwise Condensation Experiment A—Nucleation and Shedding Comparison

In addition to CAH, the dropwise condensation heat transfer depends on anumber of complex factors including nucleation site density andpopulation distribution. To investigate the behavior of these surfacesduring condensation, saturated pure water vapor at 800 Pa was condensedwhile cooling the surface with a Peltier device to a supersaturation of1.16±0.05 and imaging with an environmental scanning electron microscope(ESEM). 2 mm×2 mm sample substrates were secured to an aluminum stubwith double-sided carbon adhesive and instrumented with a K-typethermocouple embedded into the tape. The aluminum stub was clamped intoa Peltier cooling stage (Coolstage Mk 2, Deben) which was attached tothe stage of an environmental scanning electron microscope (EVO 55,Zeiss). The chamber was purged with water vapor three times up to 3 kPaand down to 10 Pa to remove non-condensable gases. After purging, thepressure was held at 800 Pa, and the temperature was slowly decreased ata rate of 0.5 K min⁻¹ until formation of observable water droplets (>1μm diameter). Accelerating voltage was 20 kV and beam current was 100nA. Images were recorded at ca. 1 Hz, and the stage was moved todifferent areas to avoid charging effects on nucleation. Nucleationdensities were measured as the mean and standard deviation of at leastfive different locations on each surface. During the pre-coalescencegrowth regime, it was observed that the nucleation density on ap(PFDA-co-DVB) surface (173±19 mm⁻², as shown in FIG. 2a ) wassignificantly higher than that on a fluorosilane surface (110±10 mm⁻²,as shown in FIG. 2b )—owing at least in part to the rougher surfaceproviding a larger number of concavities that act as nucleation sites.During condensation of an air stream saturated with water vapor underambient conditions (21° C., 40% relative humidity), the departingdiameter was 2.0±0.3 mm (as illustrated in FIG. 2c ). This isconsiderably smaller than the departing drop sizes on other commonhydrophobic modifiers such as gold (3.3 mm) and oleic acid (4.3 mm).When compared to a silanized silicon surface with a departing diameterof 2.9±0.2 mm (as shown in FIG. 2d ), a shift was also observed in thedistribution of droplet diameters to smaller sizes (as shown in FIG. 2e). The increased nucleation density, lower departure diameter, anddroplet size distribution of the copolymer surface on a smooth siliconsubstrate indicate an improved condensation heat transfer coefficientaccording to widely-accepted models.

Dropwise Condensation Experiment B—Aluminum Substrate Experiment

Commercial condensers are typically constructed using alloys of metalssuch as titanium, stainless steel, copper, and aluminum. To test aprototype that was most similar to an industrial condenser, a 40 nm filmof p(PFDA-co-DVB) was grafted onto 50 mm diameter aluminum substratesvia iCVD. The additional roughness imparted by the metal surface(RMS=118±33 nm) was apparent in the AFM height scans (shown in FIGS. 3aand 3b ). As expected on a rougher surface in a Wenzel state, the CAHmeasured during condensation at 6.9 kPa was similar (37°±5°) andaccordingly, the size of a departing drop (4.2±0.1 mm) was larger thanthat on a silicon substrate (as shown in FIG. 3c ).

Dropwise Condensation Experiment C—Effect of Grafting

In this Example, coated substrates were tested for condensationperformance in the apparatus described below and shown in FIG. 6. Theflow loop of the test apparatus is shown in FIG. 7. Saturated steam isproduced by an electric boiler using deionized feedwater with aresistivity of 5 MOhm that is further passed through a membrane vacuumdegassifier to reduce dissolved oxygen to below 1 ppm. The steam isproduced at 380 kPa and passes through a pressure regulator and aseparator to the condensing chamber, which is evacuated before each testby a rotary vane vacuum pump. The sample is cooled by a heat exchangeroperating at 60 psig with 1 MOhm deionized chilled water at 4° C.

Condensing specimens coated with p(PFDA-co-DVB) were secured in achamber with the coated side exposed to saturated steam and the otherside cooled by running water, in FIG. 6. The chamber was initiallyevacuated to remove non-condensable vapors, and steam was introduced ata variable rate to maintain pressures ranging from 10 kPa to 100 kPa.Saturated steam was produced by an electric boiler using deionizedfeedwater with a resistivity of 5 MOhm that was fed through adegassifier to reduce dissolved oxygen to below 1 ppm. The rear side ofthe sample was cooled by a forced chilled water at 4° C. Temperaturegradients within the specimens were measured by thermistors embedded atprecise locations within the specimen. The heat transfer coefficientcould be determined from the temperature gradient and the surfacetemperature. After several hours of operation, the coated specimensexhibited an improved heat transfer coefficient.

FIGS. 8a-8b show (a) Grafted PFDA and (b) ungrafted PFDA samples after 1hour of condensation in saturated steam at 90° C. and 70 kPa. FIGS.8c-8d also show condensate drops on (c) grafted and (d) ungrafted PFDAsurfaces after 10 minutes of condensing saturated steam. The distorteddrop shape on the ungrafted sample indicates severe contact line pinningfollowing delamination of the polymer film. Departing drop sizes onungrafted sample were 3.1 mm, compared to 2.3 mm for the graftedsurface. Heat transfer coefficient was measured at 31±2 kW/m²K atbeginning of test, 23±2 kW/m²K after deterioration of ungrafted surface.This example illustrates how covalent grafting can significantly improvethe adhesion of the polymer films on metal substrates and increase theirdurability in the presence of condensing steam.

Dropwise Condensation Experiment D—Film Thickness & Heat TransferCoefficient

Referring now to FIG. 1a , monomer and initiator species are flowed intoa reactor at controlled rates, where the monomer and initiator speciesencounter heated filaments and a cooled substrate, as shown in FIG. 1b .The locally heated zone around the filaments thermally cleaves theinitiator species (tert-butyl peroxide, TBPO). The produced radicalfragments initiate vinyl polymerization of the monomers absorbed on thesurface, which is held at a lower temperature. The functional groups,such as the perfluorinated side chain of PFDA, are fully preserved afterpolymerization.

The film thickness is measured in-situ during deposition, so that theprocess can be stopped when the thickness reaches the desired value. Insome embodiments, the iCVD copolymer layers are ultra-thin (˜40 nm),leading to an estimated contribution to total thermal resistance of lessthan 0.001%. To verify that the film thickness did not have an effect onthe condensation heat transfer coefficient, two different thicknesses offilms were measured, with the results being provided in Table 6 below.As seen in Table 6 below, the condensation heat transfer coefficients ofthe two film thicknesses are nearly identical.

TABLE 6 Effect of Film Thickness on Heat Transfer Coefficient Thickness(nm) h (k W m⁻² K⁻¹) 41.5 ± 2.4 38.1 ± 4.0 59.2 ± 6.6 39.5 ± 4.2

Dropwise Condensation Experiment E—Prolonged Exposure Experiment

Accelerated endurance tests were conducted by condensing pure saturatedsteam at 100° C., Coatings of p(PFDA-co-DVB) were compared tofluorosilane coatings, both on aluminum substrates (shown in FIGS. 3cand 3d ). FIG. 3e shows a comparison of these two surfaces, along withan uncoated aluminum surface that undergoes filmwise condensation forreference, under prolonged condensation at 103.4 kPa. Although thesilanized surface initially displayed a larger heat transfer coefficientof 61±2 kW m⁻² K⁻¹ due to the lower hysteresis (31°±3°) and departingdroplet size (3.6±0.4 mm), it quickly degraded in a matter of minutesand exhibited dropwise condensation with a heat transfer coefficient of4.6±0.4 kW m⁻² K⁻¹. The grafted polymer coating exhibited dropwisecondensation with a departing droplet size of 4.2±0.1 mm and a heattransfer coefficient greater than 35 kW m⁻² K⁻¹, which was more than 7times greater than the steady-state filmwise heat transfer coefficientof the degraded silanized surface, with no noticeable degradation after48 hours of condensation.

Grafted polymers deposited via iCVD lead to robust dropwise condensingsurfaces that can sustain prolonged exposure (e.g., >48 hours) to steamat 100° C., significantly outperforming a fluorosilane treatment testedunder identical conditions. A simple first-order exponential to fit tothe degradation of the heat transfer coefficients results in degradationtime constants of ca. 2 minutes and O≈10⁴ hours for fluorosilane andgrafted copolymer surfaces, respectively. Thermal degradation of filmsdeposited using the iCVD process has previously been tested anddescribed by a logistic model. Since degradation under a steamenvironment is an entirely different process, and fitting to thelogistic model would require knowledge of the time required to degradedto 50%, there is a further need for longer-duration endurance tests. Theunique composition of the copolymer achievable iCVD is essential forachieving low contact angle hysteresis, which results from thecombination of low roughness and limited reorientation of the surfacefluorinated groups between the wet and dry states. iCVD surfaces exhibitheat transfer coefficients that are more than 7 times greater thanfilmwise condensation when deposited on practical engineering heattransfer substrates, such as aluminum and copper. A successfulindustrial prototype has been demonstrated and successfully tested,indicating scalability to industrial processes.

Dropwise Condensation Experiment F—Tubing Coil Experiment

As a further demonstration of the versatility of iCVD-depositedcopolymers to coat complex shapes such as heat exchanger tubing, a 40 nmthin film of p(PFDA-co-DVB) was grafted conformally onto the outersurface of a copper tubing coil. It would have been exceedinglydifficult to achieve such an ultra-thin, uniform layer by common surfacemodification techniques such as spray coating, spin casting and/ordoctor blade application, and/or with vacuum techniques such assputtering and/or evaporation. As shown in FIGS. 4a-4b , the tubing coilexhibited prolonged dropwise condensation after a single-stepdeposition.

Dropwise Condensation Experiment G—Hydrocarbon Condensation Experiment

To demonstrate the ability of a grafted iCVD surface to promote dropwisecondensation of low-surface tension fluids, a silicon substrate coatedwith a thin film of p(PFDA-co-DVB) was fixed in a custom-built vacuumchamber shown in FIG. 6 such that the surface was held vertically.Hydrocarbon vapors were supplied by a container filled with 30 mL ofeither pentane or hexane, and immersed in a water bath (Julabo FP-25)heated to 40° C. The vacuum chamber was purged three times below 0.1 kPaand above 50 kPa with pentane vapor to remove non-condensables. Afterpurging, the rear side of the surface was cooled with forced chilledwater to a temperature of around 10° C. The hydrocarbon vapor pressurewas increased by opening a needle valve until the correspondingsaturation temperature was greater than 10° C., thus initiatingcondensation of hydrocarbon vapor onto the chilled copolymer surface.FIGS. 20a and 20b show snapshots of dropwise condensation of hexane on acopolymer film. Hexane CAH and departing diameter are also shown in FIG.20b . Heat transfer coefficients during condensation of pentane vaporwere measured by thermistors embedded behind the surface. Thecondensation heat transfer coefficient of pentane was 22.5 kW/m²K,condensing at a pressure of 52.0 kPa saturation temperature 17.7° C.,surface temperature 17.4° C., and a heat flux of 7.3 kW/m².

EQUIVALENTS

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method of preparing a surface of a condenser,the method comprising the step of performing hot wire chemical vapordeposition (HWCVD) to graft a polymeric film on the surface of thecondenser, wherein the polymeric film has a thickness no greater than1500 nm and wherein the polymeric film has a surface with low contactangle hysteresis of no greater than 50° for water.
 2. The method ofclaim 1, wherein the step of performing HWCVD comprises performinginitiated chemical vapor deposition (iCVD) to produce the polymeric filmgrafted on the surface of the condenser.
 3. The method of claim 1,further comprising the step of annealing the polymeric film by exposureto heat (e.g., to increase crosslinking density and/or degree ofcrystallinity of the polymeric film).